With Lepton, what pixel
values does the PT1-Y16 return for imaging the sun and blue sky?

Y16 is a USB video format.

The sun subtends an angle of about 0.5 degrees, and a pixel on
Lepton 2.5 has an iFOV of 0.6 degrees, so the sun cannot completely
fill a pixel. Most likely the image would only be partially filling two
or more pixels, so not presenting the full radiation of the sun on any
one pixel, although it would probably still saturate these pixels.
Assuming that Radiometry is on, the camera is in high resolution
T-linear, and is in high gain mode, 45,000 counts would be 177 degrees
C, which would probably be saturation for those pixels unless the
camera is very cold. The actual sun is hotter, of course.

A value of 23,000 assuming the same configuration would
indicate -43 degrees C, which is also about where the camera would be
expected to saturate, unless the camera is very hot. The actual deep
sky would be colder.